This week the Guardian ran a special investigation into police surveillance of protestors, and the controversial term "domestic extremism". The main articles are linked here, although there are more on their site.

Doth I protest too much?
When police admit you could be put on a secret database for being at a demo, it's time to worry.

Policing demonstrations: Grounds for protest
"The real cause for alarm is that instead of being balanced by competing priorities of the public, the force's natural desire to extend its knowledge about and control over protests is being reinforced by changes in both technology and the law."